They are all variations of the lead acid battery.
The wet cell comes in two types; serviceable and maintenance free. Both are filled with electrolyte and are basically the same. Serviceable batteries have removable caps so the electrolyte can and should be checked regularly, they are easier to test as you can check the specific gravity with a hydrometer of every cell. Maintenance free batteries are sealed for life and the water level does not need to be maintained, they are usually calcium batteries. They also have a built in hydrometer giving some indication of the charge in one cell.
AGM and Gel batteries are both types of Valve Regulated Lead Acid (VRLA).
AGM batteries are similar to wet batteries, except the electrolyte is being held in the glass matts, as opposed to freely flooding the plates. For more benefits see our Fullriver Product Advantages. A gel batteries electrolyte is gel like and immobile.
There is some common confusion regarding AGM batteries because different manufactures call them by different names they are also known as “dry cell”, “sealed lead acid” and “non spillable”, usually when consumers ask for a gel cell they mean AGM . Gel cells are still being sold but AGM batteries are replacing them in most applications.
When being recharged AGM and Gel cell batteries require different voltages than conventional batteries. If using the batteries in a deep cycle situation you must have a charger designed for AGM of Gel batteries. For a one off recharge a normal charger will do the job but the battery must not overheat so it should be closely monitored.